One example of an apparatus that uses an aqueous solution is a fuel cell powerplant. Fuel cell powerplants produce electric power by electrochemically consuming a fuel and an oxidant in one or more electrochemical cells. The oxidant may be pure oxygen or a mixture of gases containing oxygen, such as air. The fuel may be hydrogen. One source of hydrogen is a fuel processor that reforms natural gas or any appropriate hydrocarbon by using heat and steam to crack hydrocarbons.
Typically, a stack of fuel cells is used in performing the electrochemical reaction. During the electrochemical reaction, the fuel cell stack produces electric power, a reactant product and waste heat. A cooling system removes waste heat from the stack. The cooling system may advantageously use an aqueous coolant to provide both waste heat and water (as steam) to the fuel processor.
The cooling system includes flow paths for the aqueous coolant which are bounded by conduits. The conduits extend to the steam separator and to the fuel cell stack for ducting the coolant to critical locations. These conduits may have small orifices for controlling the distribution of coolant throughout the cooling system.
One problem with aqueous coolants is the cumulative deposition of particles on the walls of conduits. The particles may occur as ions or as minute parts of matter. The particles, which are capable of accumulation to the point of blockage, are generally iron based compounds. These iron based compounds are composed mainly of iron based oxides, such as magnetite and hematite, iron based salts, such as iron phosphates, and other compounds which result from the corrosion of iron including certain ferric hydrous oxide particles (hereinafter type I ferric hydrous oxide). The iron based compounds may form as the coolant comes in contact with materials containing iron. Such contact might occur as the coolant is flowed through conduits in the powerplant or through supply conduits to the powerplant.
The problem is particularly troublesome for cooling systems using small orifices because the particles may block the orifices. Any blockage of an orifice in a fuel cell stack, for example, will increase the flow resistance through the stack and may even cause an inadequate supply of coolant to a critical location within the fuel cell stack.
One way of establishing the effect of such deposition on flow resistance through the stack is to treat the supply conduits of the fuel cell stack as if they were an equal number of equally sized ideal orifices. The diameter of these equally sized orifices is called the equivalent diameter of the fuel cell stack. The equivalent diameter may be found experimentally as follows:
1. Establish a constant flow rate of coolant through the fuel cell stack.
2. Measure the pressure drop through the fuel cell stack.
3. Calculate an equivalent diameter for the stack (or any system) by the equation EQU De=2.8 (W.sup.2 /.DELTA.PN.sup.2 d).sup.1/4 10.sup.-7
where: PA1 De=Effective Diameter, inches; PA1 W=Total Coolant Flow (pound per cubic foot); PA1 d=Coolant Density (pounds per cubic foot); PA1 N=Number of Conduits in Cell Stack; PA1 .DELTA.P=Differential Coolant Pressure Across Cell Stack (pounds per square inch, difference).
The above method may be used with any system having an interior by modeling the system with a conduit which simulates the temperature pressure and other physical conditions which are present.
FIG. 2 is an example of the effect such depositions can have on the equivalent diameter of a fuel cell stack under actual operative conditions. After twenty-five hundred hours of operation (curve A) the actual equivalent diameter Dea was less then seventy percent of the initial equivalent diameter Dei. The decrease in size and the resulting decrease in the flow rate of coolant required a shutdown of the fuel cell stack for cleaning.
The fuel cell stack was cleaned after this period of operation by flowing a pressurized, acidic solution through the conduits. Cleaning restored the actual equivalent diameter Dea to ninety-five percent of the initial effective diameter Dei (curve B). After another twenty-two hundred hours of operation (curve B), the actual equivalent diameter decreased to less than seventy percent of the initial equivalent diameter. Again, both the decrease in size and the reduced coolant flow rate required the shutdown of the fuel cell stack for a second cleaning. The fuel cell stack was also shutdown for other reasons at 3,700 hours of operation (curve B) before the second cleaning. After restarting the powerplant, the equivalent diameter recovered for a short period (approximately 250 hours) before decreasing again. It is theorized that the recovery is connected with transient conditions in temperature and flow rate which occur during a shutdown and start-up of the fuel cell stack. As shown, the effect is temporary.
The powerplant was cleaned again for a second time. After less than sixteen hundred hours of operation (curve C), the effective diameter decreased to almost seventy percent of the initial equivalent diameter.
These periodic shutdowns and cleaning operations which result from particle deposition are both time consuming and costly.
Several approaches have been suggested for solving the problem of particle deposition from an aqueous coolant. One suggested approach is to reduce the amount of particles (including particles that are ions) by providing a purified aqueous coolant, suppressing corrosion by raising the coolant's pH to high levels consistent with materials used in constructing the system and by reducing the dissolved oxygen levels below forty parts per billion (40 ppb).
Chemical additives are used in highly contaminated solutions to promote the formation of sludge which is periodically removed from the system.
Ironically, cleaning out the sludge formed by the chemical additives is a common cause for service.
Another suggested approach for reducing the amount of particles is to use an aqueous coolant having a pH of about 6 to 8. Moderate levels of dissolved oxygen are permitted in the water (40-400 ppb) to suppress corrosion. Chemical additives are generally avoided.
Each of the above-mentioned methods utilizes a controlled flush rate from the system, called blowdown, which is necessary because corrosion cannot be totally eliminated and chemical cleaning is eventually required. The aqueous coolant that is lost is replaced by adding coolant. The added coolant is commonly called feedwater.
As will be realized, these techniques are equally applicable to the task of suppressing corrosion in high temperature apparatuses that use aqueous solutions such as boilers which raise steam. Boilers and other high temperature water systems experience corrosion and deposition problems which require extensive control of the water chemistry to achieve reliable system service. General industry experience has shown that even with the best efforts towards maintaining the water chemistry results are varied and repairs and/or cleaning is eventually necessary.
Despite the existence of these techniques for suppressing corrosion and for controlling the amount of iron based compounds in cooling systems having an aqueous coolant, scientists and engineers are seeking to develop additional ways of suppressing corrosion and directly blocking the severe deposition of iron based compounds on the walls of conduits.